This invention relates to remotely controlled switching apparatus which is particularly useful in an oil well or the like for performing downhole switching operations.
There are many industrial installations in which it is desirable to perform switching operations at a remotely disposed, and often inaccessible, location. In an oil well, for example, it is desirable to measure various physical parameters in the downhole environment, such as temperature and pressure, and it is common practice to employ the cable which carries electrical power to a downhole submersible pump motor for carrying signals between a downhole instrumentation package which monitors the desired physical parameters and a control and readout unit on the surface. The downhole motor may be a three-phase Y-connected AC motor, and DC signals may be employed for controlling the downhole instrumentation package and for monitoring the downhole parameters.
In such oil well installations, it is also desirable to measure periodically the motor/cable system insulation to ground resistance, since this gives an early indication of impending motor or cable failure. Such measurements require the use of relatively high voltages, e.g., 500 volts or more. Since the downhole instrumentation package transducers and their associated circuits generally have a much lower impedance to ground than the motor/cable insulation system, it is necessary to disconnect the downhole package from the cable prior to testing and to reconnect the downhole package when testing has been completed. Remotely controlled downhole switching apparatus is employed for this purpose.
One such switching apparatus is disclosed in U.S. Pat. No. 4,178,579 to McGibbeny et al, issued Dec. 11, 1979, and assigned to the same assignee as the present invention. In the McGibbeny et al apparatus, the downhole instrumentation package is connected to the neutral of a three-phase motor by a magnetically biased reed switch. The switch may be opened by applying to the cable a negative voltage of a predetermined magnitude which causes a Zener diode to break down and allows current to flow through a delatch coil. The magnetic field produced by the delatch coil is sufficient to overcome the field of a permanent magnet which holds the switch in closed position once it is closed. To close the switch, a latch coil, whose magnetic field adds to that of the permanent magnet, is energized by rectified AC current flowing in the secondary of a current transformer which has as its primary one of the three-phase lines to the motor.
Typical oil well downhole temperatures run approximately 200.degree. C., which has made it difficult to achieve good reliability and good MTBF (mean time between failures) in downhole switching apparatus. At such temperatures, semiconductor components, such as Zener diodes, are almost at their junction breakdown temperature (typically 300.degree. C.), and are a good portion of the way toward the melting temperature of silicon (approximately 600.degree. C.).
Although the McGibbeny et al switching apparatus performs its switching functions well, it has several disadvantages. The hold-off voltage at the Zener diode in the McGibbeny et al switching apparatus must be relatively high to prevent unwanted delatching by AC offset voltages at the motor neutral, and since the current required for delatching is only about 30 percent less than the absolute maximum rated current of the Zener diode, the power which must be dissipated by the Zener diode during delatching is quite high. Accordingly, delatching time must be limited, or the Zener diode must be cooled, to prevent its failure. In addition, the leakage current through the Zener diode increases with temperature and produces an error in the readout of the downhole instrumentation package. Although the leakage current can be reduced by reducing the junction area of the Zener diode, this reduces the power dissipation capability of the Zener diode and increases the possibility of a junction breakdown.
It is desirable to provide switching apparatus which overcome these and other disadvantages of known switching apparatus, and it is to this end that the present invention is directed.